Borehole investigating apparatus of the induction logging type having a slotted metal support member with coil means mounted thereon



y 3, 1966 M. M. A. GOUILLOUD 3, 9,858

BOREHOLE INVESTIGATING APPARATUS OF THE INDUCTION LOGGING TYPE HAVING ASLOTTED METAL SUPPORT MEMBER WITH c011. MEANS MOUNTED THEREON Filed Oct.11, 1962 2 Sheets-Sheet 1 In 35K 4 ATTORNEY y 3, 1966 M. M. A. GOUILLOUD3,249,858

BOREHOLE INVESTIGATING APPARATUS OF THE INDUCTION LOGGING TYPE HAVING ASLOTTED METAL SUPPORT MEMBER WITH COIL MEANS MOUNTED THEREON Filed Oct.11, 1962 2 Sheets-Sheet 2 M/rfie/ Mar/e A/er 2 60 u///0 ua INVENTOR.

ATTOIP/VE} United States Patent BOREHOLE INVESTIGATING APPARATUS OF THEINDUCTION LOGGING TYPE HAVING A SLOT- TED METAL SUPPORT MEMBER WITH COILMEANS MOUNTED THEREON Michel Marie Albert Gouilloud, Paris, France,assignor to Societe de Prospection Electrique Schlumberger, S.A., Paris,France, a corporation of France Filed Oct. 11, 1962, Ser. No. 229,904

4 Claims. (Cl. 3246) This invention relates to borehole investigatingapparatus and, particularly, to such apparatus which is adapted toinvestigate subsurface earth formations which have been penetrated by aborehole drilled deep into the earth.

In seeking to determine the presence and depth of hydrocarbon-bearingzones (oil, gas, etc.) that may exist in the subsurface earth formationspenetrated by a borehole drilled into the earth, various types ofinvestigating apparatus are frequently lowered into the borehole formeasuring various properties of the formations adjacent the borehole. Inthe course of such investigations, the investigating apparatus issubjected to high fluid pressures (e.g., 17,000 p.s.i.) at hightemperatures (e.g., 390 F.). Consequently, such investigating apparatusmust be designed and manufactured with special care for Withstandingthese severe conditions.

One form of borehole investigating apparatus that has been heretoforeproposed is described in U.S. Patent No. 2,964,698, granted to A. E.Lehmberg, Jr., on December 13, 1960. In the illustrated embodiment, thisapparatus employs transmittingv and receiving coils for inducingelectrical current flow in the subsurface earth formations and formeasuring the magnitude of such current flow. Apparatus of this type iscommonly referred to as induction logging apparatus.

One of the problems with induction logging apparatus is to protect thetransmitting and receiving coils from the destructive effects of thedrilling fluid contained in the borehole and this problem is aggravatedby the high temperature and high pressure of the drilling fluid. Thedesired protection is obtained in the Lehmberg apparatus by mounting thecoil system on a central support member and then providing afluid-impermeable sleeve member which encloses both the coil system andcentral support member, the ends of the sleeve member being secured tothe ends of the central support member in such a manner as to establisha fluid-tight seal therebetween. Means are also provided for equalizingthe pressures inside and outside of the fluid-tight housing formed bythe sleeve member.

As a consequence of this form of construction, the transmitter andreceiver coils are completely protected from the drilling fluid. Also,since the pressures sustained inside and outside the fluid-impermeablesleeve -member are always balanced, the apparatus can be used for longperiods of time without experiencing any mechanical failure thereof. Inaddition, with this form of construction the apparatus can be easilydisassembled. This enables a relatively easy and accurate positioning ofthe various coils and also enables any repairs required of such coils tobe made relatively easily and relatively quickly.

While the foregoing apparatus has been proven to provide satisfactoryand advantageous operation in boreholes of conventional size (eight inchdiameter), it has been found to be rather difiicult to constructapparatus of this type for use in small diameter (three inch) boreholes.A major problem is that the portion of the apparatus containing thecoils is, of necessity, approximately feet in length. Consequently, whenthe radial dimensions of the apparatus are reduced to provide a smalldiameter, on the order of three inches or less, it is found that themechanical strength and stifiness of the apparatus is not great enoughto enable satisfactory manipulation of the apparatus in the borehole.

It is an object of the invention, therefore, to provide new and improvedborehole investigating apparatus for investigating subsurface earthformations.

It is another object of the invention to provide new and improvedinduction logging apparatus of a fluid-tight character.

It is a further object of the invention to provide new and improvedinduction logging apparatus which is adapted for use in small diameterboreholes.

In accordance with the invention, a borehole investigating apparatuscomprises a supporting head and an elongated longitudinally-slottedmetal support member having one end secured to the supporting head. Theapparatus also includes coil means secured to the support member overthe slotted region thereof. The apparatus further includes an elongatedsleeve member enclosing the support -member and coil means and havingone end engaging the supporting head. The apparatus further includessecuring means engaging the other end of the sleeve member and securedto the support member.

For a better understanding of the present invention, together with otherand further objects thereof, reference is had to the followingdescription taken in connection with the accompanying drawings, thescope of the invention being pointed out in the appended claims.

Referring to the drawings:

FIG. 1 is a partly cross-sectional view showing a representativeembodiment of borehole investigating apparatus constructed in accordancewith the present invention;

FIG. 2 is a cross-sectional view taken along the section line 22 of FIG.1;

FIG. 3 is an enlarged view of one of the coils of the FIG. 1 apparatus;

FIGS. 4 and 5 are schematic representations of different possiblerelation-ships between portions of the FIG. 1 apparatus; and

FIGS. 4A and 5A are cross-sectional views taken along the correspondingdesignated section lines of FIGS. 4 and 5.

Referring to FIG. 1 of the drawings, there is shown a representativeembodiment of borehole investigating apparatus 10 constructed inaccordance with the present invention for investigating subsurface earthformations adjacent. a borehole drilled into the earth. The boreholeinvestigating apparatus 10 includes an elongated generally-cylindricalcentral support member 11 and an outer elongated cylindricalfluid-impermeable sleeve member 12 which encloses the support member 11.The upper end of the central support member 11 is secured to agenerally-cylindrical fluid-impermeable supporting head 13. Supportinghead 13 is provided with an outer flange portion 14 encircling thecircumference thereof. The upper end of sleeve member 12 engages theshoulder formed by flange portion. 14 and the lower part of support head13. A plurality of coil means are secured to the central support member11 intermediate the two ends thereof. in the present embodiment, thesecoil means includes a transmitter coil 15 and a .pair of receiver coils16 and 17 which are spaced apart above and below the transmitter coil15.

The apparatus 10 also includes securing means 18 en gaging the lower endof the sleeve member .12 and secured to the support member 11. Thesecuring means 18 includes a circular fluid-impermeable mounting ring 19having upper and lower circular collar portions 20 and 21 which are setin from the outer edge of the mounting ring 19. The lower end of sleevemember 12 engages the shoulder formed by the upper collar portion 20 and3 the main body of the mounting ring 19. Mounting ring 19 is alsoprovided with a center passageway 22 having a diameter which provides asnug fit with the outer surface of the support member 11 and yet is notso tight or permanent that the mounting ring 19 cannot be movedlongitudinally on the support member 11, provided the appropriate forceis utilized. The securing means 18 also includes a resilient spring-likemember of considerable stifiness in the form of a Belleville washer 23.Washer 23 is of conical shape and encircles the support member 11. Thesecuring means 18 further includes an internally-threaded'nut 24 whichis threaded onto a threaded lower portion 25 of the support member 11.

As seen in FIG. 2, the central support member 11 is provided with acenter passageway 26 which extends from one end of the support member 11to the other. Over the middle portion of the support member 11, thiscenter passageway 26 is connected to the region exterior to the supportmember 11 by means of a lateral passageway provided in this embodiment,by a longitudinal slot 27 which extends completelyacross the diameter ofthe support member 11 and which intersects the center passageway epoxyresin.

26 as indicated in FIG. 2. Asseen in FIG. 1, the slot 27 does not extendin a longitudinal direction completely to the ends of support member 11.It terminates a short distance on the outer sides of the outermost coils16 and 17. The lower end 28 of the central pasageway 26 is internallythreaded for purposes which will be explained hereinafter.

Flexible fluid-impermeable means represented by a bellows member 29 issecured to the lower end of the central support mmeber 11. This isolatesthe bottom opening of center passageway 26 from the surroundingenvironment and completes the fluid-tight enclosure about coils 15, 16and 17. The bellows member 29 is constructed of non-magnetic materialsuch as a non-magnetic metal. A nose piece 30 is secured to the mountingring 19 around the periphery of the lower collar portion 21. Two largepassageways 31 and 32 are provided in the sides of the nose piece 30.

At the upper end of the investigating apparatus 10, an elongatedfluid-tight housing section 34, only a portion of which is shown, issecured to the upper side of the supporting head 13. This fluid-tighthousing 34 is adapted to house various electrical circuits which areused to operate the coil system. Also, in practice, coupling means areusually provided for connecting and disconnecting the housing 34 and theinvestigating apparatus 10 from one another, though, for sake ofsimplicity, such coupling means are not shown in the drawings. In thisrespect, the supporting head 13 constitutes part of the investigatingapparatus 10 and remains intact therewith for the case where a couplingmeans is provided.

Coil systems are extremely sensitive to the presence of metallicmaterialsespecially those which exhibit appreciable magneticpermeability. Consequently, the investigating apparatus 10 should beconstructed with a minimum of metal material and any such material whichis used must be selected and located with extreme care. It is desirable,however, to make the central support member 11 out of a metal materialin order to increase the mechanical strength of the apparatus.Consequently, in the present embodiment; the support member 11 is madeof a non-magnetic metal. A particularly suitable metal for this purposeis austenitic stainless steel because this material has god mechanicalstrength and is essentially nonmagnetic in character. The supportinghead 13, the mounting ring 19, the conical washer 23 and the nut 24 arealso made of non-magnetic material such as this same austeniticstainless steel.

The sleeve member 12, on the other hand, must be made of a materialwhich is non-conductive as well as non-magnetic in order to preventundue attenuation of the electromagnetic energy passing from and to thecoil system. It must, nevertheless, possess a high degree of mechanicalstrength. Consequently, the sleeve member 12 is constructed of a toughplastic material such as a laminated fiber glass cloth materialimpregnated with Nose piece 30 is also constructed of this same plasticmaterial.

The interior of the fluid-tight chamber formed by the sleeve member 12as well as the interior of the bellows member 29 is completely filledwith a suitable silicon oil. This oil is selected such that, when theinvestigating apparatus is descending in the borehole, expansion of theoil with temperature is largely offset by the crushing of the bellows 29due to the increasing pressure so that the volume of the oil as a wholeremains substantially constant. The quantity of the oil used, thethermal coefficient of expansion and the coefficient of compressibilityof this oil, as well as the stillness and volume of the bellows member29, are selected to accomplish this purpose. In order to adapt thevolume of the oil to the volume of the chosen bellows, suitable spacermembers of plastic material and of appropriate size may be mounted onthe central support member 11 intermediate the coils 15, 16 and 17. Forsake of simplicity, these are not shown in the drawings.

The interior of the apparatus 10 is filled with the silicon oil by wayof a passageway 35 extending through the supporting head 13, a suitableplug 36 being used to seal 011? the passageway 35 after the apparatus 10is filled.

In order to further increase the mechanical strength and durability ofthe investigating apparatus 10, the outer sleeve member 12 is placed ina highly prestressed condition during the construction thereof andbefore the apparatus is used in a borehole. This is accomplished bymeans of a hydraulic jack which, for simplicity, is not shown in thedrawings. In particular, with both the nose piece 30 and the bellowsmember 29 removed from the remainder of the apparatus, a jack rodintegral with the piston of the hydraulic jack is screwed into thethreaded portion 28 of the passageway 26 and the cylindrical jack barrelis seated on the periphery of the mounting ring 19.

The jack is then operated to apply several thousand pounds oflongitudinal pull on the central support member 111 relative to themounting ring 19 and, hence, the sleeve member 12. For the particularmaterials mentioned above, this longitudinal force is on the order of22,000 pounds. The mounting ring 19 is capable of undergoinglongitudinal movement under the influence of large forces of this sort.Consequently, the central support member 11 is stretched and the outersleeve member 12 is compressed, both in the longitudinal direction, carebeing exercised not to exceed the elastic limits of either the supportmember 11 or the sleeve member 12.

While the apparatus 10 is maintained in this stressed condition, nut 24is tightened on the threaded portion 25 until a substantial crushing orcompression of the conical washer 23 is produced. Once this adjustmentis completed, the pressure applied to the hydraulic jack is removed,t-he piston rod of the jack is unscrewed from the support member 11 andthe jack as a whole removed from the investigating apparatus. Bellowsmember 29 and nose piece 30 are then secured to the investigatingapparatus in the appropriate manner. The investigating apparatus is thenfilled with silicon oil by way of the passageway 35 in supporting head13. The investigating apparatus 10 is then. ready for use ininvestigating a subsurface borehole.

As the investigating apparatus 10 is lowered into a borehole, thethermal expansion of the central support member 11 with temperature issomewhat greater than the thermal expansion of the sleeve member 12. Theresilient character of the conical washer 23, however, serves to preventany large changes in the prestressed condition of the apparatus.

Any bending of the apparatus 10 at the level of flange 14 would, in theabsence of the prestressing, produce tensile stresses on the fibers onone side of the sleeve memher 12. However; the prestressed condition ofthe sleeve member 1*2'is sufficient for any such bending that willnormally be encountered so that the fiberson the tensile side of thesleeve member 12 actually remain in a state of compression.- In otherwords-,- the tensile forces due to bending never exceed the prestressedcompressive forces. Under such conditions, there is always a fluidtightsealing engagement at the level of flange 14 even though the upper endof sleeve member 12 is only resting against this flange portion; Also,since the stresses sustained by the sleeve member 12 are always of acornpressive nature, the materials composing the sleevemember 12 arespecially chosen so as to be able to withstand compression forces.

The prestressed condition of the support member 11 and sleeve member12serve to ensure a fluid-tight seal at the two ends of the sleevemember 12. Also, since the internal and external pressures on the sleevemember 12 are balanced by the pressure equalization provided by thebellows member 2 9, no dangerous pressure differentials are createdbetween the inside and outside of' the sleeve member 12 and, hence, thissleeve member can be of a relatively thin and lightweight construction.This pressure balance also enables the passageway 22 in the mountingring 19 to provide a fluid-tight seal between the mount ing ring 19 andthe supportmem-ber 11 without any further precautions being required.

At the upper end of the apparatus 10, it is necessary that theelectrical conductors or wires connecting the various coil elements tothe electrical circuits contained within the upper housing 34 passthrough the supporting head 13. This is achieved by means of suitablefluid-tight passageways through the supporting head 13. Depending on theconstruction of the upper housing 3-4 and its manher of coupling to theinvestigating. apparatus '10, it may be necessary that these conductorpassageways be constructed to withstand substantial pressuredifferentials. This, however, presents no appreciable problem.

Since the metal used in the central support member 1 1 and other metalparts of the investigating apparatus are of a non-magnetic character, nolarge disturbances are produced in the alternating magnetic field set upby the transmitting coil '15 during the operation of the apparatus.Nevertheless, since these metal parts are some- What conductive incharacter, certain precautions must be exercised in order to avoidundesired spurious currents in these metal parts which would undesirablycouple with the receiver coils.

One type of precaution which can be taken is in the construction of thecoils themselves.- This is illustrated in connection with FIG. 3 of thedrawings which shows an enlarged View of the transmiter cot-1'15. Asseen FIG. 3, the coil '15 includes an inner base layer 40, which iscomposed of plastic material, a second layer 41 of plastic materialbearing an electrostatic shield 41a and a third layer 42 of plasticmaterial having a number of grooves or channels cut therein forcontaining the electr-ical conductors or wires which form the coilwinding. A fourth layer comprising an outer eleetro'static shield isalso included but, for sake of simplicity, has been omitted from theview of FIG. 3. i

The system of grooves cut into the outer surface of the coil supportinglayer 42 includes a plurality of circular grooves 43 individuallyencircling the layer 42 and a plurality of angular grooves 44interconnecting adjacent ones of the circular grooves 43. A singlelongitudinal groove 45 is provided for running a conductor from the topof the coil form to the lowermost circular groove 43. This longitudinalgroove 45 is cut deeper than the other grooves. Also, it is located sothat it crosses the angular grooves 44 at their midpoints. A continuousinsulated conductor wire 46 is placed in these various grooves in themanner indicated in FIG. 3.

The conductor wire 46 consequently includes circular 6 portions 47 whichconstitute the significant portion of the coil as far as the desiredexternal investigations are concerned.- These circular portions or turnsare interconnected by linear portions 48 lying in the angular connectinggrooves 44. A longitudinal connecting portion 49 is provided in thelongitudinal groove 45. The upper ends of the conductor wire 46 passthrough a passageway 50 in the various layers 40, 41 and 42 down to thecenter of the support member 11. At this point, the ends of conductorwire 46 are connected to suitable lead-in wires 51 and 52 which runthrough the passageway 26 in the middle of the support member 11.

The receiver coils 16 and 17 are constructed in a similar manner.

The manner in which spurious currents in the metal parts of theapparatus are prevented from occurring will now be considered.Electrical current flowing in the circular portions 47 of thetransmitter coil 15 would tend to set up circular current new in thebody of the support member 11, concentric with the longitudinal axisthereof. This, however, cannot occur in the present apparatus because ofthe presence of the slot 27.

The linear connecting portions 48 and the longitudinal connectingportion 49 of the coil 15 also tend to induce spurious currents in thesupport member 11. This tendency, however, is considerably reduced bythe fact that the longitudinal conductor portion 49 is placed in veryclose physical relationship with the linear conductor portions 48 andwill in the operation of the apparatus, carry current in a directionopposite to the current flowing in the linear portions 48. Nevertheless,this compensation is not complete and some spurious currents will stillexist in the support member 11. The significance or" these remainingspurious currents depends upon the orientation of the linear conductorportions 43 with re- .spect to the slot 27. This is explained withreference to FIGS. 4, 4A, 5 and 5A of the drawings.

FIGS. 4 and 4A illustrate the worse possible positioning of the linearconnecting portions 48 with'respect to the slot 27. In this case, theplane of the slot .27 (axis 2782 of FIG. 4A) is at right angles to aradial'plane passing through the mid-points of the linear connectingportions 48 (axis 48a of FIG. 4A). As represented in FIG. 4, thisorientation enables a circulating current flow, represented by dash line5-5, in the body of the support member 1 1. This is because the magneticfield set up by the linear connecting portions 48 is asymmetricallylocated relative to the two sides or legs of the support member '11.Consequently, the magnetic field strength is greater in the nearer legand smaller in the farther leg. This difference in field strengthenables a longitudinal current flow down. one leg and up the other legof the support member 11, as indicated by dashed line 55, the endportions of support member 11 completing the electrical circuit. Sincethis current flows through each of the receiver coils 1'6 and 117 in aparallel direction with respect to the general longitudinal trend of thelinear connecting portions 48, some spurious voltage is induced in theseconnecting portions and, hence, appears across the terminals of thereceiver coils.

The orientation shown in FIG. 5, on the other hand, represents thepreferred orientation of the linear. connecting portions relative to theslot 27. In this case, the plane of the slot 27 coincides with the planepassing through the mid-points of the linear connecting portions 48. Inthis case, while the residual magnetic field associated with the linearconnecting portions 48 is still asymmetrical with respect to the centerof the support member 11, it is nevertheless now symmetrical withrespect to the two legs of the support member 11. Consequently, theelectromotive forces induced in the two legs of the support melmber 11are in the same direction and have identical magnitude distributions asa function of distance from the mid-points of the linear connectingportions 48. Consequently, these induced electromotive forces opposeeach other as far as any current flow circulating up one leg member anddown the other is concerned. A localized flow of current, as indicatedby dashed line 56, will occur in each of the legs of support member 11in the immediate vicinity of the transmitter coil 15. These currents,however, are of a localized nature and do not couple with the receivercoils 16 and 17.

In the construction or re-assembly of the investigating apparatus 10,before the sleeve member 12 is put in place, the coil system isenergized with electrical current and the coils 15, 16 and 17, which areinitially positioned as indicated in FIG. 5, are slowly rotated back andforth by small amounts to determine in a precise electrical manner theexact optimum orientation of the coils with respect to the slot-27.

From the foregoing description of the invention, it is seen thatborehole investigating apparatus constructed in accordance with thepresent invention represents apparatus possessing a high degree ofrigidness and mechanical strength even though the apparatus is ofrelatively great length and small diameter. Also the coils containedwithin the apparatus are completely protected against the destructiveeffects of fluids contained in the borehole and this protection ismaintained even though the apparatus is subjected to relatively severemechanical stresses.

While there has been described what is at present considered to be thepreferred embodiment of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein Without departing from the invention and it is, therefore,intended to cover all such changes and modifications as fall Within thetrue spirit and scope of the invention.

' What is claimed is:

1. Induction logging apparatus for movement through a borehole drilledint-o the earth for investigating subsurface earth formations traversedby such borehole comprising: a supporting head adapted for movementthrough the borehole; an elongated cylindrical metal support memberhaving one end secured to the supporting head and having alongitudinally-extending slot out completely through the body of thesupport member completely across a diameter thereof to divide thesupport member into two completely separate parts over the longitudinalextent of the slot; a cylindrical coil coaxially mounted on the supportmember over the slotted region thereof; an elongated cylindricalnon-magnetic sleeve member enclosing the support member and the coil andhaving one end engaging the supporting head; and securing means engagingthe other end of the sleeve member and secured to the support member.

. 2. Induction logging apparatus for movement through a borehole drilledinto the earth for investigating subsurface earth formations traversedby such borehole comprising: a supporting head adapted for movementthrough the borehole; 'an elongated metal support member having one endsecured to the supporting head and having a longitudinally-extendingslot cut into the body thereof; at least one coil secured to the supportmember over the slotted region thereof, this coil comprising a pluralityof nearly complete circular turns positioned in parallel planes with theends of adjacent turns interconnected by linear conductor segments withthe midpoints of the linear conductor segments lying in the plane of theslot; an elongated sleeve member enclosing the support member and coiland having one end engaging the supporting head; and securing meansengaging the other end of the sleeve member and secured to the supportmember.

3. Induction logging apparatus for movement through a borehole drilledinto the earth for investigating subsurface earth formations traversedby such borehole comprising: a supporting head adapted for movementthrough the borehole; an elongated longitudinally-slotted metal supportmember having one end secured to the supporting head; coil means securedto the support member over the slotted region thereof; an elongatedsleeve member enclosing the support member and coil means and having oneend engaging the supporting head; and securing means engaging the otherend of the sleeve member and secured to the support member at a relativelongitudinal location' that creates and maintains several thousandpounds of longitudinal compressive stress in the sleeve member.

4. Induction logging apparatus for movement through a borehole drilledinto the earth for investigating subsurface earth formations traversedby such borehole comprising: an upper supporting head adapted formovement through the borehole; an elongated cylindrical metal supportmember having an upper end secured to the supporting head and having alongitudinally-extending slot out completely through the body of thesupport member completely across a diameter thereof to divide thesupport member into two completely separate parts over the longitudinalextent of the slot; at least one cylindrical coil coaxially mounted onthe support member over the slotted region thereof, this coil comprisinga plurality of nearly complete circular turns positioned in parallelplanes with the ends of adjacent turns interconnected by linearconductor segments with the midpoints of the linear conductor segmentslying in the plane of the slot; an elongated cylindrical nonmagneticsleeve member enclosing the support member and the coil and having anupper end engaging the supporting head; and lower securing meansengaging the lower end of the sleeve member and secured to the lower endof the support member.

References Cited by the Examiner UNITED STATES PATENTS Bravenec et al.324-6 WALTER L. CARLSON, Primary Examiner.

G. R. STRECKER, Assistant Examiner.

2. INDUCTION LOGGING APPARATUS FOR MOVEMENT THROUGH A BOREHOLE DRILLEDINTO THE EARTH FOR INVESTIGATING SUBSURFACE EARTH FORMATIONS TRAVERSEDBY SUCH BOREHOLE COMPRISING: A SUPPORTING HEAD ADAPTED FOR MOVEMENTTHROUGH THE BOREHOLE; AN ELONGATED METAL SUPPORT MEMBER HAVING ONE ENDSECURED TO THE SUPPORTING HEAD AND HAVING A LONGITUDINALLY-EXTENDINGSLOT CUT INTO THE BODY THEREOF; AT LEAST ONE COIL SECURED TO THE SUPPORTMEMBER OVER THE SLOTTED REGION THEREOF, THIS COIL COMPRISING INPLURALITY OF NEARLY COMPLETE CIRCULAR TURNS POSITIONED IN PARALLELPLANES WITH THE ENDS OF ADJACENT TURNS INTERCONNECTED BY LINEARCONDUCTOR SEGMENTS WITH THE MIDPOINTS OF THE LINEAR CONDUCTOR SEGEMENTSLYING IN THE PLANE OF THE SLOT; AN ELONGATED SLEEVE MEMBER ENCLOSING THESUPPORT MEMBER AND COIL AND HAVING ONE END ENGAGING THE SUPPORTING HEAD;AND SECURING MEANS ENGAGING THE OTHER END OF THE SLEEVE MEMBER ANDSECURED TO THE SUPPORT MEMBER.